Since an ITO thin film containing 2 to 20% by mass of tin oxide has a high electric conductivity and an excellent light transmission performance, it is employed as a transparent conductive film for a transparent electrode of a liquid crystal display.
An ITO thin film is produced mainly by a spattering method using an ITO target, and a sintered article produced by molding and sintering an ITO powder or by molding and sintering a powder mixture of an indium oxide powder and a tin oxide powder is employed as an ITO target.
One of the problems encountered when forming a transparent conductive film by a spattering method using an ITO target is a low availability of the ITO target.
An ITO sintered article as an ITO target undergoes a reduction in its mass in response to its history of being used in the spattering. In general, the quality of a transparent conductive film is ensured by replacing an ITO target with a new one at a reduction in the mass of the ITO sintered article within the range of 20 to 80% by mass. Since the ITO sintered article as a wasted ITO target (ITO sintered article scrap) contain a substantial amount of indium which is a rare resource and expensive, a highly pure elemental indium or indium compound is recovered from such wasted ITO target.
For example, Japanese Unexamined Patent Publication No. 3-75224 discloses a method for separating tin and indium by dissolving a wasted ITO target in hydrochloric acid, preparing an aqueous solution containing indium and tin, adding ammonium ion or sodium ion and then generating a halogenostannate. However, this method is unsuccessful in separating metals other than tin from indium.
Japanese Unexamined Patent Publication No. 3-82720 discloses a complicated method comprising reducing an ITO scrap solution, adjusting pH from 2 to 5 to precipitate an indium component as a hydroxide, dissolving the hydroxide in an acid, allowing indium to be adsorbed on a non-chelate ion-exchange resin and then allowing the adsorbed indium to be desorbed.
An object of the invention is to provide a simple method for producing an indium-containing aqueous solution having a reduced amount of metal impurities.